GCC strength reduction bug
See the message below. It seems that the old a.out compiler in the
stable tree has this problem. What are we going to do about it ?
Ian.
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Newsgroups: comp.os.linux.announce
Message-ID: <cola-liw-821042267-8219-1@oravannahka.helsinki.fi>
From: albert@krakatoa.ccs.neu.edu (Albert Cahalan)
Subject: gcc bug reminder
Date: Sun, 07 Jan 96 19:17:47 GMT
- -----BEGIN PGP SIGNED MESSAGE-----
/* sr_bug.c
*
* This program tests for a gcc bug.
* To compile this test program: gcc -O2 sr_bug.c
*
* Sometimes gcc for Intel CPUs generates bad code at optimization
* level 2. The bad code is 'almost right' and stay hidden inside
* a program that seems to work - including the Linux kernel.
* The bug is very old and has been reported. As of 19-Dec-95,
* the bug has not been fixed.
*
* If you change this code you should test it, because even a
* tiny change will hide this elusive bug. If you think you
* fixed the bug, please run the original test just to make
* sure. You can find the original test below, after the #if 0.
* I wrote this version of the test to be user-friendly, and
* it may not be as solid as the original.
*
* Some people who know more than I do:
* davis@space.mit.edu (John E. Davis)
* anlauf@crunch.ikp.physik.th-darmstadt.de (Harald Anlauf)
* craigs@iii2.iii.net (Craig Shrimpton)
*
* User-friendly version by Albert Cahalan
*
*/
#include <stdio.h>
int gcc_sr_bug(void){
static int Array[3]; /* must be static (or global) */
unsigned int B = 3; /* must be unsigned 32-bit */
int i;
for(i=0; i<B; i++) Array[i] = i - 3;
for(i=0; i<B; i++) printf(" %d,%d", i, Array[i]);
return !Array[1];
}
int main(){
printf("Testing for gcc bug...");
if(gcc_sr_bug()){
printf("\n\nBad code! Your compiler generates bad output.\n\n");
printf("Add -fno-strength-reduce to your gcc command line\n");
printf("or put it into your gcc config file, such as in\n");
printf("/usr/lib/gcc-lib/i486-linux/2.7.0/specs.\n");
exit(1);
}else{
printf("\nOK, no problem.\n");
exit(0);
}
}
#if 0
/********* original code + whitespace ***********/
#include <stdio.h>
int A[3];
unsigned int B = 3;
void printit(void){
int i;
for(i = 0; i < B; i++) fprintf(stdout, "A[%d] = %d\n", i, A[i]);
}
int main(){
int i;
for(i = 0; i < B; i++) A[i] = i - 3;
printit();
return 0;
}
#endif
- - --
Albert Cahalan
albert@ccs.neu.edu
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